To avoid thermal shorting, the MTPV system is preferably operated in a vacuum enclosure or housing H which enables an evacuated gap G; and gap spacers—made, for example, of silicon dioxide—are employed to set the gap between the emitter and the photovoltaic cell receiver in a manner which minimizes heat transfer through the spacers. Phonons or non-radiated energy carriers are a source of inefficiency though they transfer energy from the source; but they do not have the individual energy to excite electrons across the bandgap.
As described in the above referenced paper, a previous method of forming the spacers between the heat emitter and the photovoltaic cell substrate was to grow a thick oxide on the emitter chip and pattern the oxide through such methods as photolithography and plasma etching into cylindrical spacers, with the spacers to be about six microns in diameter; but a disadvantage of this technique is that the spacers permit too large a heat loss from the emitter, reducing the efficiency of conversion of heat to electricity and increasing the cooling requirements on the photovoltaic cell.
Another disadvantage arises in the use of micrometer gap thermophotovoltaic devices of large area, requiring, for example, brazing individual chips to create a “tiled” surface as, for example, in U.S. Pat. No. 6,232,546. A single large emitter chip and photovoltaic cell cannot be used because the emitter is operated at about 1000° C. and the photovoltaic cell must be kept at room temperature to function effectively as a collector of photons and a generator of electrons. The difference in thermal expansion between the heater and the photovoltaic cell as the heater chip is heated from room temperature to of the order of 1000° C., can break the spacers or distort the geometry during the temperature excursion if there is such a rigid attachment.
An approach to solve this problem is to use an array of laterally spaced hollow tubes of thermally resistant material disposed in wells formed in the heat emitter substrate, each carrying a flange on top and serving as a spacer extending into the gap—as indicated at S in the drawings—a structure that lends itself to fabrication by established microfabrication methods such as lithography and plasma etching, particularly with a silicon emitter substrate and silicon dioxide spacers.
A more facile and less complicated and less costly construction is now, however, provided by the present invention, as later described in detail.